Speaker cables, what's true and what's not.

Hoi,
Ok, I understand that a speaker cable  is not a normal 220V cable.
But can anyone calculate what the thickness of a speaker cable should be.
In my opinion it has to do something with Speaker resistor (8 ohms),
Voltage? and Current?, probably used frequency.
So how do I determine what the thickness of a cable should be, or should I
use Bi-wiring.
Please help me,
Harry.